Methods for deposition of materials in underground reservoirs

ABSTRACT

A method of precipitating or depositing material within an underground reservoir which comprises introducing into the reservoir in aqueous solution (i) an isolated enzyme and (ii) a substrate for the enzyme, such that the action of the enzyme on the substrate leads to the precipitation or deposition of material within the underground reservoir.

The method of this invention is generally applicable to the control offluid movement in underground reservoirs through the reduction of theporosity or permeability of the geological formation. The method isespecially suitable for use in the recovery of oil and gas fromhydrocarbon containing reservoirs.

During oil production operations, a range of problems are encounteredarising from the unwanted breakthrough of an overlying gas body, or anedge or bottom water, to the production well by coning or channelling.This is a particular problem where reservoir heterogeneities such asfractures or high permeability streaks are selectively depleted of oil,allowing the premature entry of adjacent gas or water into theproduction zone. In heavy oil reservoirs, channelling or fingering ofwater through the relatively immobile oil phase can result in loss ofheavy oil production.

A range of methods have been employed in order to increase the recoveryof oil from underground reservoirs. In one form of enhanced recovery, adrive fluid is injected under pressure into the oil reservoir throughone or more injection wells to maintain, restore or produce formationpressure. The most widely used drive fluid is water. More complexaqueous systems, such as those containing polymer or surfactant, orother fluids such as solvents or gases may also be used. Steam may beused for heavy oils. The drive fluid is often introduced into theoil-bearing underground formation near the bottom of the formation at orabove reservoir pressure, to displace oil in the formation. As the fluidmoves through the reservoir, it drives or flushes the oil through theformation. An increasing oil saturation develops ahead of the movingfluid and finally reaches the production well or wells. Generally, anoil-bearing underground formation will consist of various regions havingdifferent permeabilities. Drive fluid moves preferentially through theregions of higher permeability and in so doing, bypasses oil containedin much lower permeability regions. This obviously reduces the sweepefficiency of the displacing medium.

The flow of fluids through the formation may be modified to improve theproduction of oil. Reducing the permeability of selected regions canreduce coning, channelling or fingering or improve the sweep efficiencyduring primary, secondary or enhanced production.

A number of approaches have been proposed to reduce permeability.Processes which use crosslinked polymers or other types of gels havebeen most common. Other processes using foams, emulsions, suspendedsolids, microorganisms and precipitates have also been proposed(Seright, R. S. & Liang, J.; Paper SPE 30120 A Comparison of DifferentTypes of Blocking Agents. pp. 431-440 In Proceedings of the EuropeanFormation Damage Control Conference, May 15-16, 1995, The Hague, TheNetherlands). A number of these processes use hazardous chemicals.Thermal or bacterial degradation of the blocking agent may occur.

The precipitation or deposition of materials within the formation mayarise from mixing two or more incompatible chemical solutions in theformation or selectively removing a chemical or chemicals which keepother chemicals in solution. If the process occurs rapidly, however,placement of the precipitate can be difficult.

Ferris and Stehmeier (U.S. Pat. No. 5,143,155) teach that bacteria maybe used to precipitate minerals from an aqueous system. Growth of thebacteria on nutrients is required before the minerals are precipitated,allowing some time to place the fluid. However, bacterial systems sufferfrom a number of potential disadvantages. Nutrients must be supplied.These may be used by organisms other than the intended species orstrains, either introduced or indigenous. The bacteria must grow underthe reservoir conditions of temperature, pH and salinity. These areoften sub-optimal for the preferred organisms. The efficiency ofconversion of growth nutrients to desirable products is often low.Bacteria may produce different metabolic products to those intended. Thedegree of control over the system, including the rate at whichprecipitation occurs is limited. In addition, bacteria may not readilyenter anything other than a high permeability formation due to theirsize.

Acidising of underground reservoirs using a combination of esterase orlipase enzymes and esters has already been described (PCT/GB94/00922,PCT/GB95/01295). The use of the produced acid to precipitate or depositother chemicals was not taught.

The present invention teaches the use of enzymes to precipitate ordeposit materials within an underground reservoir. Preferably theunderground reservoir is a hydrocarbon, for example gas or oil, or waterreservoir. The method of precipitating or depositing chemicals withinunderground reservoirs comprises introducing into the reservoir inaqueous solution (i) an enzyme and (ii) a substrate for said enzyme,such that the action of the enzyme on the substrate leads to theprecipitation or deposition of material within the undergroundreservoir.

The material which is precipitated or deposited may be present, in wholeor in part, in the reservoir before the introduction of the enzyme andthe substrate.

Alternatively, the material is precipitated or deposited from an aqueoussolution or dispersion (iii) introduced into the reservoir in additionto the enzyme and substrate. It is preferable but not essential to usean aqueous solution or dispersion (iii).

It is necessary to select an enzyme which remains active under reservoirconditions. The following parameters are generally taken intoconsideration:

1) Temperature Tolerance

The temperature of a reservoir is a function of its depth and can be inexcess of 100° C. Many onshore reservoirs and some offshore reservoirsin carbonate formations are fairly shallow with temperatures fallingwithin the 30-60° C. range. Generally the enzymes used in the method ofthe present invention are active between 15° C. and 110° C., for examplebetween 15° C. and 95° C. but an enzyme which is active at highertemperatures may also be used. The enzymes used in the process of theinvention have a range of temperatures over which they are active. Whenthere is a temperature gradient in the oil/gas well, it may be desirableto use two or more enzymes together to ensure reliable operation overthe temperature range within the well.

2) Pressure Tolerance

Pressure is also a function of depth. Pressures in offshore reservoirsin, for example, the North Sea may exceed 500 atmospheres, whereasshallower on-shore fields are likely to be in the range 50-150atmospheres. If enzymes are to be injected at rates above fracturepressure, they must withstand injection pressures which will exceedreservoir pressure.

3) Salt Tolerance

The ability to withstand high salt levels is important as reservoirbrines can often be near saturated solutions. Enzymes may be injected infresh water, but they will need to withstand the effects of saltsdiffusing into that fresh water.

4) Oil Tolerance

Enzymes must be tolerant of oil although they may remain in the aqueousphase within the reservoir.

The enzyme used in the method of the present invention is generally awater soluble enzymrte. It is advantageous for the enzyme to be readilywater soluble. Preferably the enzyme is a hydrolase (EC 3) such as alipase (EC 3.1.1.3), an esterase (EC 3.1.1.1) or a urease (EC 3.5.1.5)or an oxidoreductase (EC 1.) such as an oxidase or peroxidase.

Typically isolated enzymes are used. Enzymes may be isolated from plant,animal, bacterial or fungal sources. The enzymes may be produced fromwild-type, conventionally bred, mutated or genetically engineeredorganisms. The enzymes may, optionally, be chemically modified, as longas they retain or possess a desired catalytic ability. Individualenzymes are selected for their ability to act on the selected substrate,producing a desired change under the conditions of the undergroundreservoir. Preferably, the enzymes will be industrial enzymes availablein bulk from commercial sources.

The substrate is generally a chemical substrate. The substrate for theenzyme and other materials required for the process will normally betechnical grade to reduce the cost of the process.

Enzyme-substrate combinations which are considered to be particularlyuseful for isolated enzyme based deposition processes are: esterases orlipases plus esters; ureases plus urea; phosphatases plus organicphosphates; oxidases or peroxidases plus phenols; and oxidases orperoxidases plus anilines.

Preferably the aqueous solution or dispersion (iii) comprises a salt ofNa, Ca, Si, Mg, Al, or Fe such as calcium chloride, sodium bicarbonate,ferrous sulphate, ferric chloride, aluminium chloride, aluminiumsulphate, magnesium chloride, colloidal dispersions of silica or anorganic compound capable of forming a resin or gel or a polymer capableof being crosslinked to form a gel and a crosslinking agent or a mixturethereof. The presence of a metal salt may, in alkali conditions, resultin the formation of one or more metal hydroxides.

The precipitated or deposited material is typically a mineral, a gel, ora resin. Examples of each of these are provided. We do not wish to belimited to these examples. Other combinations of enzymes, substrates andaqueous solutions or dispersions (iii) which may result in theprecipitation or deposition of materials will be apparent to thoseskilled in the art.

Suitable combinations of enzyme and substrate will depend on theprevailing conditions in the reservoir. For example, it may not besuitable to use an acid producing combination of enzyme and substrate,in the presence of acid soluble material such as carbonate. The acidwould react with the carbonate and the pH of the solution would remainhigh. This may prevent the precipitation or deposition of materials,particularly if the precipitation or deposition requires acidicconditions. This is more likely to be the case in carbonate formationsor in sandstone formations where significant amounts of carbonateweighted drilling fluids have been used. Possible limitations to theoperation of individual systems will be apparent to those skilled in theart and will help determine the choice of system for the particularconditions encountered.

The solution or solutions of enzyme, substrate and additional chemicalsmay be prepared in suitable water for example city (drinking) water,produced water, fresh water (for example water from lakes, rivers orponds) or seawater. The solutions may be prepared batchwise in tanks orother suitable vessels or prepared by adding these components to thewater on a continuous, preferably controlled and monitored basis (“onthe fly”) as the water is injected into the reservoir.

Suitable concentrations of substrate and the material present in theaqueous solution or dispersion (iii) (if used) will depend on therequired amount of precipitation or deposition. This will depend on thespecific system chosen but will typically be of the order of 1 to 50grams per litre, although higher or lower concentrations may beappropriate in some situations. The enzyme concentration will beselected to produce precipitation or deposition within the desiredperiod of time for particular enzyme-substrate-additional chemicalcombinations. Typical enzyme concentrations will be 0.0001% to 2% v/v ofcommercial liquid enzyme preparations, preferably 0.001 to 1% v/v or theequivalent activity of dried enzyme preparation.

Solutions of the enzyme, substrate and the aqueous solution ordispersion (iii) are conveniently introduced into the undergroundreservoir via injection or production wells.

They will normally be introduced at below fracture pressure but may beinjected at above fracture pressure. A single solution containing all ofthe components may be used or more than one solution containingindividual components or two or more components may be used. More thanone enzyme-substrate-aqueous solution or dispersion (iii) combinationmay be used at one time, if compatible.

Oxidases and peroxidases useful in the process of the present inventionrequire either molecular oxygen (dioxygen) or a peroxide as an electronacceptor. Suitable enzymes include horseradish peroxidase, soybeanperoxidase, chloroperoxidases, haloperoxidases, lactoperoxidase,oxidases, laccase and tyrosinase. Preferably molecular oxygen or amolecular oxygen containing gas is used as an electron acceptor with anoxidase and a peroxide is used as an electron acceptor with aperoxidase. Peroxides known to be useful as electron acceptors forperoxidases include hydrogen peroxide, alkyl peroxides such as ethylperoxide or methyl peroxide, aromatic peroxides and peroxy acids.

When one or more electron acceptors are needed they can conveniently beintroduced into the underground reservoir via injection or productionwells. They may be introduced as a solution containing molecular oxygenor a peroxide or as a solution or dispersion containing compounds whichdecompose to liberate molecular oxygen or peroxides.

Suitable compounds include perborates, percarbonates, perphosphates,persilicates, hydrogen peroxide adducts such as urea hydrogen peroxideand magnesium peroxide. Molecular oxygen or molecular oxygen containinggases or peroxides or other required reactants or precursors of thereactants may be introduced as a foam.

When hydrogen peroxide is required for use with a peroxidase, it may begenerated from molecular oxygen using an oxidase enzyme or enzymesintroduced into the underground reservoir together with a suitablesubstrate or substrates. Suitable combinations of substrates and oxidaseenzymes include glucose plus glucose oxidase (EC 1.1.3.4), urate plusurate oxidase (EC 1.7.3.3), galactose plus galactose oxidase (EC1.1.3.9), alcohols plus alcohol oxidase (EC 1.1.3.13), amines plus amineoxidase (EC 1.4.3.4, EC 1.4.3.6) and amino acids plus amino acid oxidase(EC 1.4.3.2, EC 1.4.3.3).

The solutions may also be injected sequentially, with or without spacerfluids. Some mixing of reservoir water and injected aqueous solutionswill occur in the reservoir. In optimising the systems for givenreservoir conditions, the composition of the reservoir water and thewater used to make up the solutions to be injected (for example city(drinking) water, produced water, fresh water (for example water fromlakes, rivers or ponds) or seawater) may be taken into consideration.Individual waters may contribute significant amounts of an ion requiredfor a particular precipitation or deposition process.

The treatment fluids introduced into the formation may optionallycontain materials which act as a focal point for nucleation. This mayassist in the precipitation of minerals such as calcium carbonate fromthe supersaturated solutions formed as a result of the action of theenzymes on their substrates.

The well may be shut in after introduction of treatment fluid or fluidsor injection or production from operations continued. Ordinarily, iftreatment fluids are introduced into an injector well, injection offluid into the well will be continued. If the fluid is intended for sandconsolidation, the well may be shut in for a period of time, typicallybetween 1 hour and a week, preferably 6-48 hours, to allow effectiveconsolidation. If the fluid is intended to precipitate or depositproduction chemicals in the near wellbore vicinity, a similar shut inperiod may be required.

Enzymes have a number of advantages over bacteria for the controlledproduction of chemicals in oilfield environments. Suitable enzymepreparations often have several months shelf life at ambienttemperature. Their efficiency of conversion of substrates to productsmay be very high. The kinetics of production can be accuratelycontrolled in contrast to systems which depend on the growth of bacteriawhere lag times can generally vary. There is no requirement for growthnutrients to be provided for enzyme-based systems. Growth nutrients areoften a costly component of systems based on bacteria. Introduction ofgrowth nutrients into a reservoir may encourage the growth ofundesirable organisms. Enzymes can be used in the presence of certainbiocides. The conditions under which enzymes can operate are in generalmore extreme than those tolerated by bacteria. Enzymes are thereforemore suited to oilfield operations. For example, industrial enzymes areknown which are tolerant of temperatures up to 110° C., extremes of pHfrom about 2 to about 12 and saturated salt solutions.

The rate of production of materials by enzymes can be accuratelycontrolled, and the system can be manipulated in a variety of ways suchthat precipitation or deposition of chemicals occurs within a given timescale. This allows the fluid to be placed in the target zone beforeprecipitation or deposition occurs. Ways in which the system can becontrolled include varying the concentration of enzyme, varying theconcentration of substrate, varying the concentration of the materialpresent in the aqueous solution or dispersion (iii), encapsulation ofthe enzyme to give a controlled release and the incorporation of varyingquantities of buffer to maintain the pH within a given range for apredetermined period of time in systems where acid or alkali productionwould otherwise result in precipitation or deposition of chemicals.

The precipitation or deposition of material may be caused by a change inthe pH of the solution. For example, a lipase or esterase enzyme incombination with an ester produces an acid which reduces the pH of thesystem. In the presence of an aqueous solution containing phenol andformaldehyde this reduction in pH can result in the formation of aphenol-formaldehyde resin. In the presence of a slurry of calciumsilicate acid conditions will destabilize the slurry and lead to theformation of a gel. On the other hand, a urease enzyme in combinationwith urea produces ammonia which increases the pH of the system. If thesolution contains sodium bicarbonate and calcium chloride the increasein pH causes the precipitation of calcium carbonate. If the solutioncontains guar and borate the increase in pH causes a gel to form. If thesolution contains at least one suitable metal salt the increase in pHcauses at least one metal hydroxide to form. In the presence of smectiteclays such as bentonite, keolinite or montmorillonite the metalhydroxide may form a complex with the clay.

Alternatively, the combination of the enzyme and the substrate canproduce a product which can react with another material in the solutionto produce an insoluble product. For example, a phosphatase enzyme, incombination with calcium glycerophosphate produces an inorganicphosphate, if the solution contains calcium chloride calcium phosphateis precipitated. Oxidation of phenols by oxidase or peroxidase enzymescan produce precursors which readily polymerise to form phenolic resins.Similarly, oxidation of anilines by oxidase or peroxidase enzymes canproduce precursors which readily polymerise to form polyanilines.

In a further embodiment of the invention, the enzyme-based precipitationor deposition of material may be employed to consolidate sand. Anincrease in the mechanical strength of the formation by theprecipitation or deposition of materials may significantly reduce therisk of sand production and/or formation collapse during hydrocarbonproduction.

Another embodiment of the invention is to use the enzyme-basedprecipitation or deposition of materials to precipitate or depositmaterials such as scale inhibitors, corrosion inhibitors, paraffininhibitors, asphaltene inhibitors or similar production chemicals withinthe formation, so that low concentrations of these chemicals may bereleased at a controlled rate over a given period of time.

In many underground reservoirs sufficient precipitation or depositionwill occur if the reservoir is treated once with one enzyme/substratesystem. In other underground reservoirs it may be necessary to repeatthe treatment with a given enzyme/substrate system one or more timesand/or to use more than one enzyme/substrate system simultaneously,sequentially or separately.

The following Examples illustrate the invention.

EXAMPLE 1

A urease enzyme, in combination with urea will produce carbon dioxideand ammonia which increases the pH of the system. In the presence of anaqueous solution of calcium such as calcium chloride the rise in pHreduces the solubility of the calcium which is precipitated as calciumcarbonate.

A solution containing 20 g urea, 10 g ammonium chloride, 2.1 g sodiumbicarbonate and 2.8 g calcium chloride per litre of distilled water wasprepared. The pH was adjusted to 6.0. Jack Bean urease obtained fromSigma-Aldrich was added to 1.5 mg/ml. At 25° C. visible precipitation ofcalcium carbonate occurred after 3 minutes. The appearance of theprecipitation was followed in a spectrophotometer by measuring theabsorbance at 600 nm. The incorporation of Tris-HCl buffer at 10, 20 and30 mM delayed the onset of precipitation by approximately 1 minute per10 mM Tris HCL.

The pH at which visible precipitate formation started was approximatelypH 8. The rate of formation of precipitate was observed to beproportional to the amount of enzyme added.

The reaction rate in this example is higher than that likely to be usedin underground reservoirs. Rapid precipitation allows monitoring of thereaction using a spectrophotometer. Longer incubations would result insettling of the precipitate requiring another method of following thereaction. Reducing the amount of enzyme used would result in thereaction taking place over a longer period, say 1 to 6 hours.

This example also shows that one of the controls which can be exertedover the precipitation or deposition process is the introduction ofquantities of a suitable buffer, which delay the onset of precipitationor deposition proportional to the amount of buffer added.

EXAMPLE 2

A phosphatase enzyme, in combination with calcium glycerophosphate willhydrolyse the glycerophosphate to produce inorganic phosphate. In thepresence of an aqueous solution of calcium such as calcium chloride,calcium phosphate is precipitated.

A phosphatase enzyme was added to a solution containing 50 mM calciumchloride, and 50 mM calcium glycerophosphate. The initial solution wascompletely clear and the enzyme preparation contained no particulates.After about 20 minutes at 20° C. the solution became opaque due to theprecipitation of white fines material. The appearance of the precipitatewas followed in a spectrophotometer by measuring the absorbance at 600nm. Increasing the amount of enzyme increased the rate of deposition.

EXAMPLE 3

A urease enzyme, in combination with urea will produce carbon dioxideand ammonia which increases the pH of the system. In the presence of anaqueous solution of guar gum and borate the rise in pH results in theformation of a crosslinked gel.

A low viscosity guar/borate solution was prepared as follows. One gramof powdered guar was dissolved in 200 mls water and the pH adjusted to3.8 by drop wise addition of concentrated acetic acid. 1.25 ml of 4% v/vborax solution as added. After addition of the borate solution to theguar the pH of the guar/borate mixture stabilised at 3.7. Four grams ofsolid urea pellets were added and when these were fully dissolved the pHstabilised at 3.9. 0.05 g of powdered urease enzyme was then added. ThepH was observed to rise. Three minutes after addition of the enzyme thepH became alkaline and the guar/borate mixture had formed a gel.

The rate at which gelation occurred was proportional to the amount ofurease enzyme used.

EXAMPLE 4

An oxidase or peroxidase enzyme, in combination with a suitable electronacceptor will oxidise a phenol to a precursor which readily polymerisesto form a phenolic resin.

A solution of 0.66% phlorglucinol (1,3,5-trihydroxybenzene) was made upin Tris-HCl pH 7.1 and Sigma Horseradish peroxidase added at 60 unitsper ml. Hydrogen peroxide was added dropwise over a two hour period togive a final concentration of 0.3%. The colour of the solution changedfrom colourless to red and an orange to yellow coloured material wasobserved to coat the inside of the polypropylene test tub after leavingovernight.

EXAMPLE 5

An oxidase or peroxidase enzyme, in combination with a suitable electronacceptor will oxidise an aniline to a precursor which readilypolymerises to form polyaniline.

An aqueous solution containing 2.8% (v/v) of an aniline and 1.8%hydrogen peroxide was prepared and the pH adjusted to pH 7. Thefollowing were then added to the reaction mixture:

sufficient peroxidase enzyme to catalyze polymerization of all of theaniline overnight under acidic conditions; esterase substrate to a finalconcentration of 6% v/v; sufficient esterase enzyme to break down theester substrate and reduce the pH to below 5 within a few hours. Thereaction was carried out at room temperature and within 2 hours ofadding the esterase enzyme the pH had dropped to below 5 and anilinepolymer was evident as a fine cloudy orange precipitate.

EXAMPLE 6

A solution containing 1M CaCl₂ and 500 mM calcium glycerophosphate wasprepared. Phosphatase enzyme was added in an amount sufficient tohydrolyse the glycerophosphate over a 48 hour period. After 72 hours atroom temperature (20° C.) a gel was observed to have formed.

EXAMPLE 7

A urease enzyme, in combination with urea will produce ammonia andcarbon dioxide and increase the pH of the system. In the presence of anaqueous solution of suitable metal salts, the rise in pH will result inthe formation of metal hydroxides.

A solution containing 30 mM aluminium chloride, 30 mM magnesium chlorideand 200 mM urea was adjusted to pH 3.8 with 1 M sodium hydroxide. Ureaseenzyme was added and the pH was observed to rise. After 16 hours the pHhad risen to 9.3 and colloidal metal hydroxides were present.

What is claimed is:
 1. A method of precipitating or depositing within anunderground reservoir a material which is one of a resin, a gel, amineral and an inhibitor, the inhibitor being selected from a scaleinhibitor and an asphaltene inhibitor, wherein said method comprisesintroducing into the reservoir in aqueous solution (i) an isolatedenzyme and (ii) a substrate for said enzyme, such that the action of theenzyme on the substrate leads to the precipitation or deposition of saidmaterial within the underground reservoir.
 2. A method according toclaim 1 wherein the material is precipitated or deposited from anaqueous solution or dispersion (iii) introduced into the reservoir inaddition to (i) and (ii).
 3. A method according to claim 2 wherein theaqueous solution or dispersion (iii) comprises a component which isselected from a salt of a metal which is one of Na, Ca, Mg, Si, Al andFe; an organic compound capable of forming a resin or gel; a polymercapable of being crosslinked to form a gel and a crosslinking agent; andmixtures thereof.
 4. A method according to claim 2 wherein the enzyme isselected from an oxidase and a peroxidase, the substrate is a phenol andthe aqueous solution or dispersion (iii) contains an electron acceptor.5. A method according to claim 2 wherein the enzyme is selected from anoxidase and a peroxidase, the substrate is an aniline and the aqueoussolution or dispersion (iii) contains an electron acceptor.
 6. A methodaccording to claim 1 wherein the precipitation or deposition is causedby a change in pH of the aqueous solution.
 7. A method according toclaim 6 wherein the pH is reduced.
 8. A method according to claim 6wherein the pH is increased.
 9. A method according to claim 1 whereinthe enzyme is a hydrolytic enzyme.
 10. A method according to claim 9wherein the hydrolytic enzyme is selected from an esterase, lipase,urease and phosphatase enzyme.
 11. A method according to claim 1 whereinthe enzyme is an oxidoreductase.
 12. A method according to claim 11wherein the oxidoreductase enzyme is selected from oxidase andperoxidase enzyme.
 13. A method according to claim 1 wherein thesubstrate is a chemical substrate.
 14. A method according to claim 13wherein the chemical substrate is selected from an ester, urea, aphenol, an aniline and a phosphate containing organic compound.
 15. Amethod according to claim 1 wherein the enzyme is a urease and thesubstrate is urea.
 16. A method according to claim 15 wherein theaqueous solution or dispersion (iii) contains calcium chloride.
 17. Amethod according to claim 15 wherein the aqueous solution or dispersion(iii) contains guar gum and an agent selected from sodium tetraborate,other borates and boric acid.
 18. A method according to claim 15 whereinthe aqueous solution or dispersion (iii) contains at least one metalsalt suitable for yielding a metal hydroxide under alkaline conditions.19. A method according to claim 18 wherein the aqueous solution ordispersion (iii) contains aluminium chloride and magnesium chloride. 20.A method according to claim 1 wherein the enzyme is a phosphatase andthe substrate is calcium glycerophosphate.
 21. A method according toclaim 1 wherein the enzyme is selected from an esterase and a lipase andthe substrate is an ester.
 22. A method according to claim 21 whereinthe aqueous solution or dispersion (iii) contains phenol andformaldehyde.
 23. A method according to claim 1 wherein the enzyme is inthe form of a delayed release formulation.
 24. A method according toclaim 1 wherein the permeability of an underground reservoir is reduced.25. A method according to claim 1 wherein sand present in theunderground reservoir is consolidated.
 26. A method according to claim 1wherein an inhibitor selected from a scale inhibitor, a corrosioninhibitor, a paraffin inhibitor, an asphaltene inhibitor and mixturesthereof is deposited within the underground reservoir.